The present invention relates generally to a method and an apparatus for the flexible assembly of components and subassemblies into an assembly and, in particular, to a method and an apparatus for preventing building stresses into a final assembly.
The process of constructing a vehicle body includes forming subassemblies from individual body panel components, forming larger subassemblies from groups of smaller subassemblies, panels and other components, and forming a final body assembly from the larger subassemblies. During the construction process, the components and subassemblies must be held in fixed, predetermined positions while attachment operations such as welding and inserting fasteners are performed. Typically, the components and subassemblies are held at a work or assembly station in fixtures utilizing locators and clamps which are movable to abut predetermined points on a component surface during the attachment operation and to retract from the surface to permit the subassembly to be moved to another assembly station. Since components are manufactured within a tolerance range, the locations of the predetermined points will vary from component to component and from subassembly to subassembly. The fixtures are often adjusted in holding positions to accommodate the various locations of the predetermined points.
Fixtures for holding components and subassemblies can be classified as xe2x80x9cdedicatedxe2x80x9d or xe2x80x9cprogrammable.xe2x80x9d A dedicated fixture, also known as xe2x80x9chard automationxe2x80x9d, is designed to accommodate a single component or subassembly and to perform a specific operation or set of operations. Typically, a dedicated fixture is capable of only clamping and unclamping movements necessary to perform pre-selected operations on the component or subassembly and cannot accommodate a different component or subassembly representing a design change or a different model. In contrast, a programmable fixture is capable of a range of movements. Thus, the programmable fixture can be programmed to execute a pre-determined set of movements to accomodate a particular component or subassembly, and a different set of movements to accomodate another component or subassembly.
Many programmable positioners have mechanisms of the serial type, i.e., each link of the mechanism is serially mounted on the preceding link forming a chain of links. The links may have linear or rotary joints. Mechanisms with linear joints provide higher rigidity but are still appreciably less rigid when compared with conventional hard tooling fixtures. Programmable fixtures of the serial type are described in the U.S. Pat. No. 5,272,805 (see the FIG. 7 and the FIG. 8) which patent is assigned to the assignee of the present invention.
The present invention provides a programmable positioner with a high degree of rigidity as compared with the rigidity of hard tooling fixtures. It realizes its objective by adopting a parallel linkage mechanism formed by a plurality of linear actuators. The rigidity, or stiffness, of the mechanism is determined by the cooperative supporting structure of the parallel linkage wherein the total stiffness of the mechanism is the sum of the stiffening effect of all the links. Mechanisms of this type have been utilized for flight simulators, well known as the xe2x80x9cStewart Platformxe2x80x9dxe2x80x94Stewart, The Institute of Mechanical Engineers, Proceedings 1965-1966, pp. 371-394; and for the construction of machine tools as described, for example, in the U.S. Pat. No. 5,354,158 (also, the U.S. Pat. No. 4,988,244 and the U.S. Pat. No. 5,388,935).
One of the most flexible of fixtures is a 6-axis machine tool. A support includes a pair of spaced platforms joined by six powered and extensible legs, which may or may not be crossed, attached to the platforms by universal joints. In one form, the machine tool has an operator, such as a tool in a spindle, mounted on one of the platforms and an object, such as a workpiece, mounted on the other one of the platforms such that the work space is located between the platforms. The length of the legs is individually manipulated to vary the positions of the platforms and, therefore, the object and operator relative to each other.
The U.S. Pat. No. 4,988,244 shows a machine tool having spaced platforms for mounting a tool and a workpiece which platforms are joined by six extensible legs attached to the platforms in three pairs by clevis and trunnion joints. The pairs of legs are crossed and the process operation is performed between the platforms.
The U.S. Pat. No. 5,388,935 shows a machine tool having spaced platforms for mounting a tool and a workpiece which platforms are joined by six extensible legs attached to the platforms by universal joints. At least one of the pairs of legs is crossed and the process operation is performed between the platforms.
The parallel linkage of the above described type of fixture, wherein the process work is done between the two support platforms, is not suitable for work done above the support, such as in assembly operations. The travel restrictions dictated by such a structure are not supportive of flexible assembly wherein the assembly may be performed on relatively large parts that cannot be contained between the two supports. In another type of device, also including a pair of spaced platforms joined by six extensible legs, the tool is mounted on one of the platforms but faces away from the other platform. The object is mounted on a fixture located relative to the one platform. Thus, the other platform serves as a fixed base in order to move the tool toward and away from the object on which an operation is to be performed.
The U.S. Pat. No. 4,536,690 shows a self-propelled robot platform having a support structure which includes a base and a movable tool support joined by six extensible legs wherein the work space is outside the area between the base and the support. Position control feedback utilizes position and velocity sensors.
The U.S. Pat. No. 5,053,687 shows an articulated device having spaced top and bottom plates joined by six extensible link members with a work space outside the area between the plates. The bottom plate fixedly is supported above a base plate. The link members have top ends attached to the top plate in three pairs and bottom ends attached to six slider members by universal joints. The slider members extend through apertures in the bottom plate to engage endless screws mounted on the base plate. Each slider member includes a shock absorber and is connected to position sensor and each link member includes a force sensor. When force is applied to the top plate, the shock absorbers provide passive compliance and a control utilizes the force sensor information in a force return algorithm to calculate corrections to be applied to the position of the top plate.
Whether dedicated or programmable fixtures are utilized, the components or subassemblies are rigidly clamped on the fixtures which are designed for maximum support rigidity. If the process equipment, such as a spot welding gun, is misaligned, it tends to displace the component or components from the desired assembly location. Accordingly, the components may shift relative to each other causing inaccuracy in the assembly, or, the components are caused to stretch or compress against the rigid fixtures before they are permanently welded. Thus, a problem with existing fixtures is that the assemblies are assembled with internal stresses that could cause failure of the assemblies when in use.
The present invention concerns an apparatus for positioning an object during an assembly operation including a parallel link programmable positioning mechanism having a base plate, a spaced apart locator plate and six linear actuator links extending between the two plates and attached thereto by universal joints. The ends of the actuators are paired in a triangular pattern at each plate and the actuators do not cross one another. Thus, two actuators having one end paired at the base plate each have an opposite end paired with another actuator at the locator plate. The actuators are screw drives for changing the length of each link in a programmable manner. The work space is adjacent the locator plate outside of the space between the plates. When an operation requires corrective action due to high processing forces, the actuators can be of the ball screw type which react to forces applied to the locator plate by backdriving. Rotary position encoders can be utilized to provide motion feedback to a control system which can react to the applied force to increase the holding force for higher support rigidity or decrease the holding force for controlled compliance and a stress free assembly. Also, the actuators can be coupled mechanically to reduce the degrees of freedom and the number of motors and controls required.
In the apparatus, the base plate has an upper surface, the locator plate has upper and lower surfaces and the linear actuators each have a lower end pivotally attached to the upper surface of the base plate and an upper end pivotally attached to the lower surface of the locator plate, the lower and upper ends of each linear actuator being movable relative to one another along a predetermined linear path to move the locator plate relative to the base plate. A plurality of base pivot joints are attached between the upper surface of the base plate and the lower ends of the linear actuators in a generally triangular first pattern, the base pivot joints being arranged in pairs with a midpoint of each base pivot joint pair forming a point of the first pattern. A plurality of locator pivot joints are attached between the lower surface of the locator plate and the upper ends of the linear actuators in a generally triangular second pattern, the locator pivot joints being arranged in pairs with a midpoint of each locator pivot joint pair forming a point of the second pattern.
An object is mounted on the upper surface of the locator plate. The object usually is a clamp for attaching a component to the locator plate or a tool for performing an operation on the component. A control means is connected to the linear actuators for selectively actuating the linear actuators to move the locator plate to a predetermined position relative to the base plate for positioning the object to receive a component to be assembled. A feedback means is coupled to the linear actuators and is connected to the control means. The feedback means is responsive to a force applied to the object during assembly of the component for generating feedback signals representing the applied force to the control means, and the control means is responsive to the feedback signals for actuating the linear actuators to change the applied force. The actuator can be mechanically coupled to coordinate actuation of at least two of the linear actuators to move the locator plate relative to the base plate with less than six degrees of freedom.
The present invention also concerns a method for positioning an object during an assembly operation including the steps of: connecting a base plate to a locator plate with a plurality of linear actuators each having a lower end pivotally attached to the base plate and an upper end pivotally attached to the locator plate; mounting an object on the locator plate; controlling the linear actuators to move the locator plate to a predetermined position relative to the base plate for contacting the object mounted on the locator plate with a component to be assembled; generating feedback signals representing a force applied to the locator plate when the object contacts the component; and actuating the linear actuators to change the applied force in response to the feedback signals. The method can be performed by maintaining the applied force within a predetermined threshold range.
The method according to the present invention also concerns a method for the stress free assembly of sheet metal components on assembly fixtures including at least one programmable fixture having actuators responsive to position control signals from a controller, clamping means for clamping the components to the assembly fixtures and tooling operable on and capable of displacing the components during assembly and generating forces supported by the fixtures. The method includes the steps of: generating control signals from a controller to move at least one actuator and position a fixture at a predetermined location; placing at least two components on the fixture at the predetermined location; clamping the components to the fixture; actuating tooling to assemble the components, the tooling generating forces supported by the fixture and tending to displace the components from the predetermined position; sensing the forces supported by the fixture during actuation of the tooling and generating force signals indicative of a magnitude and direction of the forces; communicating the force signals to the controller; and operating the controller to move the actuator in a direction to reduce the forces below a desirable limit to providing a stress free assembly of the components. The sensing step can be performed by a force sensor measuring at least one of six generalized force components X, Y, Z, Mx, My and Mz in a generalized cartisian coordinate system, or by a position encoder coupled to the actuator for generating an output signal representing movement of the actuator in response to the applied force.